1. Field of the Invention
The present invention relates to a waveform shaping circuit for alleviating the sharpness of the leading and trailing edges of an input pulse, thus reshaping the waveform of the input pulse so as to produce a trapezoidal pulse, and outputting the trapezoidal pulse. More particularly, this invention is concerned with a waveform shaping circuit preferable for employment in an on-vehicle multiplex communication network system.
2. Description of the Related Art
In general, for a local area network (LAN), a binary pulse (rectangular pulse) representing two levels is used as a communication signal to be transmitted over a bus line. Such a pulse (rectangular pulse) has a waveform exhibiting a sharp rise and fall, and contains many harmonic components in addition to a baseband frequency component. When the pulse is transmitted over the bus line, the induced and radiated harmonic components may adversely affect the operations of electrical equipment located nearby. If the electrical equipment located nearby includes a radio set, the induced and radiated harmonic components are mixed in and superposed on a signal received by the radio set, and thus interferes with a receiving operation of the radio set.
An on-vehicle multiplex communication network system that has been adopted widely for vehicles or the like these days is no exception to the fact that the induced and radiated harmonic components adversely affect nearby electrical equipment. When a signal propagating over a bus line is a pulse (rectangular pulse), harmonic components of the pulse whose frequencies are included in a baseband adversely affect the operations of on-vehicle electrical equipment, for example, a radio set, a television set, and a car navigation system.
For eliminating the adverse effect of the harmonic components of a pulse (rectangular pulse), as far as a multiplex communication network system, or especially, a on-vehicle multiplex communication network system is concerned, a pulse (rectangular pulse) exhibiting sharp rise and fall characteristics is not transmitted over a bus line, but the waveform of the pulse (rectangular pulse) is reshaped to produce a trapezoidal pulse exhibiting moderate fall and rise characteristics, and the trapezoidal pulse is transmitted over the bus line. Thus, the magnitude of induction and radiation of harmonic components occurring when the trapezoidal pulse is transmitted over the bus line is reduced.
For such a multiplex communication network system, several circuits have already been proposed as a waveform shaping circuit for reshaping the waveform of a pulse (rectangular pulse) so as to produce a trapezoidal pulse.
FIG. 5 is a circuit diagram showing an example of the circuitry of a known waveform shaping circuit. FIGS. 6A and 6B are signal waveform diagrams showing examples of input and output signals of the waveform shaping circuit shown in FIG. 5, FIG. 6A shows the waveform of an input pulse, and FIG. 6B shows the waveform of an output trapezoidal pulse.
As shown in FIG. 5, the waveform shaping circuit comprises a common-emitter transistor 51, a first resistor 53 and second resistor 54 connected in series between a signal input terminal 52 and the base of the transistor 51, a feedback capacitor 55 connected between the collector of the transistor 51 and a node a between the first resistor 53 and second resistor 54, a collector load resistor 57 connected between the collector of the transistor 51 and a power supply terminal 56, and an output resistor 59 connected between the collector of the transistor 51 and a signal output terminal 58.
The operations of the above circuit performed when a negative pulse (rectangular pulse) shown in FIG. 6A is applied to the signal input terminal 52 will be described below.
To begin with, when the pulse remains high, the transistor 51 is fully on. The potential at the signal output terminal 58 is therefore low. The feedback capacitor 55 has an electrode thereof on the side of the node a charged positively and has another electrode thereof charged negatively on the side of the collector of the transistor 51.
Thereafter, the pulse makes a high-to-low transition, and then remains low. This causes a current to flow from the power supply terminal 56 through the collector resistor 57, feedback capacitor 55, and first resistor 53 to the signal input terminal 52. With the current, the polarities of charges in the feedback capacitor 55 are reversed successively. At the same time, the base voltage of the transistor 51 falls gradually. The transistor 51 changes from the fully-on state through an intermediately-on stage to a fully-off state. With the change of the conducting state, the potential at the signal output terminal 58 rises slopingly so as to make a low-to-high transition, and then remains high. In this case, the sloping rise time of the potential at the signal output terminal 58 is set to an RC constant determined with a sum of resistance values of the collector resistor 57 and first resistor 53 as well as the capacitance value of the feedback capacitor 55.
Thereafter, the pulse makes a low-to-high transition and then remains high. With the high-level pulse applied to the node a through the first resistor 53 and a current flowing through a closed loop composed of the feedback capacitor 55, the collector-base passage of the transistor 51, and the second resistor 54, the polarities of the charges on the feedback capacitor 55 are reversed successively. At the same time, the base voltage of the transistor 51 rises duly. The transistor 51 changes from the fully-off state through the intermediately-on state to the fully-on state. With the change of the conducting state, the potential at the signal output terminal 58 falls slopingly so as to make a high-to-low transition, and then goes back to the initial state in which the potential remains low. In this case, the sloping fall time of the potential at the signal output terminal 58 is set to an RC constant determined with the resistance values of the first resistor 53 and second resistor 54 as well as the capacitance value of the feedback capacitor 55.
As mentioned above, the known waveform shaping circuit responds to a pulse (rectangular pulse), shown in FIG. 6A, which is applied to the signal input terminal 52. A trapezoidal pulse having sloping leading and trailing edges as shown in FIG. 6B is output through the signal output terminal 58.
The foregoing known waveform shaping circuit has the merit of low manufacturing cost because the circuit has relatively simple circuitry and consists of general-purpose circuit elements.
However, in the known waveform shaping circuit, as shown in the signal waveform diagram of FIG. 6B, the slope of the rise of a resultant trapezoidal pulse is different from the slope of the fall thereof. In particular, the slope of the fall of the trapezoidal pulse is relatively sharp. When the waveform shaping circuit is employed in an on-vehicle multiplex communication network system, the bandwidth of an output trapezoidal pulse changes due to a relatively severe voltage fluctuation occurring in an on-vehicle power supply. Depending on the changed state, the trapezoidal pulse induces and radiates harmonic components when propagating over the bus line.
Thus, the known waveform shaping circuit has a problem that assuming that the known waveform shaping circuit is employed in an on-vehicle multiplex communication network system, when a trapezoidal pulse is transmitted over a bus line, the trapezoidal pulse induces and radiates harmonic components under certain conditions.